Researchers at Kyung Hee University in South Korea have developed a platform that can charge, move and sort tiny liquid droplets using precharged polymer films that need no continuous electrical supply. The system, called SELF-EP, could simplify microfluidic devices that are used for medical diagnostics and chemical analysis.
Manipulating small droplets is central to microfluidics, a field that spans everything from disease testing to laboratory chemistry. Established methods such as electrowetting and dielectrophoresis give precise control, but they typically depend on external voltage sources and hardware that switches electrodes on and off. The new approach aims to move part of that control function into the material itself.
The films, which the team calls SELF-emitters, are charged once through a corona-charging step. After that, each film keeps its surface charge and produces a lasting electric field without being plugged into a power source. The physics relies on electrostatic induction. When a grounded liquid is dispensed near an emitter, the field drives charge redistribution and exchange with ground. The droplet then detaches carrying a net charge. Importantly, the film supplies the field rather than transferring its stored charge directly to the liquid.
By controlling the droplet charge and the polarity of the emitter, the researchers can choose whether droplets are attracted to or repelled from the film. Using this capability, the team showed that several droplets can be transported at the same time and sorted according to their charge. In another demonstration, they suspended droplets in silicone oil and moved them by manually repositioning the emitters. That experiment showed battery-free manipulation without any direct mechanical contact with the droplets.
The team also found that the surface charge of the polymer films can be patterned. These patterns form electrostatic potential-energy landscapes that cause droplets to settle into ordered arrangements and act as passive guides along predefined routes. In a long-distance routing experiment, an externally applied electric field supplied the driving force while the patterned film determined the path. According to the study, this removes the need to switch an array of control electrodes in sequence.
The findings were reported in Reports on Progress in Physics, where the authors include Donghan Lee and colleagues. The study suggests that charge-retaining materials can provide both a persistent electric field and spatial guidance for handling droplets. If the approach scales, it could support simpler and more portable microfluidic systems that rely less on powered control hardware.
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